Giant Enhancement of Third-Harmonic Generation in Epsilon-Near-Zero Material by an ITO-SiO₂ Metagrating

Due to huge electric filed enhancement resulting from the continuity of the normal component of the electric displacement field, epsilon-near-zero (ENZ) materials always exhibit a maximum of nonlinear response under large-angle oblique illuminations. As a result, the parallel components, which occupy over a half of incident power, are hardly helpful during the nonlinear processes. Here, we proposed a new approach for coupling all incident electric field into the ENZ nanostructures at the ENZ frequency. By fabricating ITO, a typical ENZ material, into a grating pattern and filling the gaps with SiO2, the incident light polarized in the direction of ITO-SiO2 periodic arrangement were totally coupled into the nanostructure under normal incidence. As a result, the electric filed in the ITO were greatly enhanced. Compared with a bare ITO film with same height at the ENZ wavelength, numerical simulations demonstrate a 103-fold THG efficiency enhancement from the proposed ITO-SiO2 metagrating under the pump intensity of I0=1.2GW/cm2. For understand the giant enhanced nonlinear effects, field distributions at ENZ frequency in the metagrating were analyzed. In addition, the angle-dependent THG efficiencies of the proposed metagrating were also investigated. The proposed ENZ-dielectric metagrating offers a reliable route towards nonlinear optical devices with low power and less space.